This paper responds to an argument of Hilary Putnam to the effect that the plurality of modern sciences shows us that any natural kind has a plurality of essences. In the past, he has argued that no system of representations, mental or linguistic, could have an intrinsic relationship to the world. Though he has granted that the Thomistic notion of form and its application to the identity of concepts may avoid these earlier objections, he has maintained that the advance of the sciences has shown us that there are too many substantial forms in any particular kind of thing to provide the unity of conceptual identity required by the Thomist’s account. Given the resemblance of Putnam’s position to the “pluralists” against whom Aquinas argued in the Summa Theologiae a consideration of Aquinas arguments is undertaken. Following this, the paper examines a particular case of recent scientific practice, in order to suggest whose position, Putnam’s or the Thomist’s, more adequately captures the practice of the natural sciences of today, and their bearing upon the metaphysical question of the nature of essence in natural kinds. The paper concludes that the Thomist position on the unity of form or essence, with qualifications made about distinct conceptual approaches to some object of investigation, and the use of analogy in sorting through these distinct approaches, is better capable of accounting for the actual goals and practices of scientific understanding as we see it practiced today than is Putnam’s transcendental nominalism and neopragmatism